arrays like [][] are called also jugged arrays. I.e., they are "arrays of arrays"
If you declare something like:
int[][] numbers = new int[2][] { new int[] {2,3,4}, new int[] {5,6,7,8,9} }; you'll have:

2, 3, 4
5, 6, 7, 8, 9

In other words, it is a bit tricky to say what are dimensions for the array... And how can you convert this array into a truly 2-dimensional array?

Say, if we want an array like [2,5] we'll have
2,3,4,0,0
5,6,7,8,9

But if we want [2,3] - we may need to throw away some array elements:
2,3,4
5,6,7

So, there is no strict recipy how to convert such arrays into each other. Obviously, convert [,] array to a jugged array much easier :)